A new species of Petrotilapia from the Nkhata Bay region of Lake Malaŵi is described. The black submarginal band in the dorsal fin distinguishes it from Petrotilapia tridentiger Trewavas and Petrotilapia chrysos Stauffer & van Snik, which lack this band. The bright blue color of the males distinguishes it from males of Petrotilapia genalutea Marsh, which have a dull blue ground color and orange flanks, and males of Petrotilapia nigra Marsh, which are predominately black with 7–10 gray/brown bars. A plot of the sheared second principal components of the morphometric data and the first principal components of the meristic data further support that these latter three species are distinct. Females of the new species are light beige to whitish laterally with 5–7 faint brown vertical bars. Juveniles are bright yellow. The new species is sympatric with P. genalutea and P. tridentiger, but underwater observations show preferences for different habitats and assortative mating among these sympatric species.